The present invention relates generally to an apparatus and method for improved reactant and coolant flow sealing within joined or fluidly-cooperating fluid-delivery plates used in a fuel cell assembly, and more particularly to the use of a microseal disposed on top of a metal bead that is integrally formed on a cooperating surface of one or both of the plates to provide more effective fluid isolation for the reactant or coolant that is conveyed through channels defined within the plate surfaces.
Fuel cells convert a fuel into usable electricity via electrochemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel cell stack.
In such a stack, adjacent MEAs are separated from one another by a series of reactant flow channels, typically in the form of a gas impermeable bipolar plate (also referred to herein as a flow field plate) that—in addition to promoting the conveyance of reactants, coolant and byproducts—provides structural support for the MEA, as well as electrical current collection or conveyance. In one common form, the channels are of a generally serpentine layout that covers the majority of the opposing generally planar surfaces of each plate. The juxtaposition of the plate and MEA promotes reactant flow to or from the fuel cell, while additional channels (that are fluidly decoupled from the reactant channels) may also be used for coolant delivery. In one configuration, the bipolar plate is itself an assembly formed by securing a pair of thin metal sheets (called half plates) that have the channels stamped or otherwise integrally formed on their surfaces. The various reactant and coolant flowpaths formed by the channels on each side typically convene at a manifold (also referred to herein as a manifold region or manifold area) defined on one or more opposing edges of the plate. Examples of all of these features—as well as a typical construction of such bipolar plate assemblies that may be used in PEM fuel cells—are shown and described in commonly-owned U.S. Pat. Nos. 5,776,624 and 8,679,697 the contents of which are hereby incorporated by reference in their entirety.
It is important to avoid leakage and related fluid crosstalk within a PEM fuel cell stack. To overcome such leakage, the Assignee of the present invention has applied a relatively thick elastomeric sealant onto discrete portions of the relatively planar surface of the bipolar plate. While useful in establishing the requisite degree of sealing, the thick nature of the sealants makes such an approach unfeasible in actual fuel cell stacks that are made up of more than one hundred bipolar plate and MEA assemblies, as volumetric concerns—especially in the confined spaces of an automobile engine compartment—become paramount. Moreover, the difficulty of ensuring a consistent, repeatable placement of the seal makes this approach cost-prohibitive.
In an alternate to using thick elastomeric sealants, the Assignee of the present invention has developed integrally-formed bipolar plate sealing where stampings formed in the plate surfaces in a manner generally similar to those used to form the reactant and coolant channels produce gasket-like outward-projecting metal beads to establish discrete contact points between adjacent plate surfaces. These beads (which may be formed to define a cross sectional rectangular, trapezoidal, semi-spherical or other related shape) are more compatible with high-volume production needs than that of the deposition of a thick elastomeric sealant such as mentioned above. In particular, the Assignee applied a thin, relatively soft, compliant sealing layer where in an ideal situation there is no variation in the thickness or structural stiffness along the length of the sealant such that the nominal sealing pressure (which is based on the applied stacking force per sealant length divided by the sealant width) should be substantially uniform. Nevertheless, proper sealing and avoidance of pressure variations along the length of the bead is difficult to achieve, especially in view of the inherent vagaries of fuel cell stack manufacturing where both dimensional tolerances of the formed beads as well as the misalignment of one hundred or more individual cells within the stack are present such that variation of seal effective pressure and concomitant leakage along the length of the bead around one or more regions of the plate is unavoidable.
An additional difficulty stems from how the sealant is adhered within the bipolar plate assembly. In the previously investigated approach by the Assignee discussed above, the sealant first forms an adhesive bond between itself and the bead substrate. As mentioned above, while conventional thick sealants tend to be relatively insensitive to such bonding, the present inventors have discovered that any attempt to reduce the thickness of the sealant results in significant sealing pressure sensitivity to how the sealant is constrained at the interface between it and the underlying substrate. For example, in the case of a 1.1 mm wide sealant that is relatively thin (i.e., about 0.15 mm high), the locations that lose adhesion or have no adhesion to begin with may exhibit pressures significantly lower than that of the same seal with perfect adhesion. A further difficulty arises out of the fact that the long service life associated with an operating fuel cell stack in a harsh automotive environment often leads to some debonding along the length of the cured sealant. Previous studies conducted by the present inventors have shown that if a spot or section loses adhesion during the fuel cell stack lifetime, that area can lose 75% of the sealing pressure, which can lead to unacceptably high levels of reactant or coolant leakage.